1. Field of the Invention
The present invention relates to a semiconductor apparatus, and more particularly, to a semiconductor apparatus which comprises at least two kinds of semiconductor devices operable by voltages of different values, i.e., a high voltage and a low voltage, on the same semiconductor substrate, and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, a silicon oxide film which has thermal resistance and enables easy setting of a manufacturing process has widely been used as, e.g., a gate insulating film of a MOS transistor. In recent years, an increased demand for shrinking of a device and higher performance has made a thickness of the gate insulating film smaller and smaller, and there has accordingly been a progress in thinning of the gate insulating film which uses the silicon oxide film. However, if the gate insulating film which uses the silicon oxide film is formed to a certain thickness or lower, a gate leakage current excessively flows, which imposes a limit on thinning.
Thus, as an alternative insulting film, a high dielectric insulating film which uses Al2O3 or Zi2O5 higher in dielectric constant than the silicon insulating film as an insulating film material has attracted attention as a thin gate insulating film candidate.
Because of its high dielectric constant compared with the silicon oxide film, such a high dielectric insulating film can defy the limit on thinning of the gate insulating film which uses the silicon oxide film, and assist manufacturing of an integrated semiconductor device including a low voltage system having a thin gate insulating film such as a logic circuit section operated by a low voltage. However, a semiconductor apparatus which handles a low voltage on one hand and a high voltage on the other hand, i.e., two or more circuit sections of different voltages on the same substrate, e.g., a memory section, an I/O section, an analog circuit section or the like of a DRAM-mounted hybrid LSI cannot use the high dielectric film effective for thinning because of presence of the device of a high voltage system to which a high voltage is applied.
Thus, recently, a semiconductor apparatus which has two or more circuit sections of the different voltages has been realized by using the conventional insulating film which uses the silicon oxide film for a semiconductor device of a high voltage system, the insulating film higher in dielectric constant than the silicon oxide film for a semiconductor device of a low voltage system, and forming both on the same substrate.
Now, description will be made of a conventional constitution of the semiconductor apparatus of the aforementioned type and an example of its manufacturing method by referring to FIGS. 4A to 4D.
In FIG. 4A, a silicon oxide film 42 is formed as an insulating film of a high voltage system on the whole surface of a semiconductor substrate 41 which includes a region 43a for forming a high voltage system device and a region 43b for forming a low voltage system device.
Then, as shown in FIG. 4B, the silicon oxide film 42 is selectively etched so as to leave a portion deposited on the region 43a of the semiconductor substrate 41 for forming the high voltage system semiconductor device and to remove a portion deposited on the region 43b for forming the low voltage system semiconductor device, and then a high dielectric insulating film 44 is deposited as a low voltage system insulating film on the whole surface.
Then, as shown in FIG. 4C, a polysilicon film 45 for forming a gate electrode is deposited on the whole surface of the high dielectric insulating film 44 formed on the semiconductor substrate 41. A resist mask is formed on the polysilicon film 45 by using a well-known photolithography method. Patterning and etching are carried out to form gate electrodes 45a, 45b shown in FIG. 4D, gate insulating films 46a, 46b1 of a double layer structure below the gate electrode 45a, and a gate insulating film 46b2 below the gate electrode 45b. 
When a semiconductor apparatus is manufactured by using such a conventional method, gate insulating films of a double layer structure which have dielectric constants different from each other, i.e., the silicon oxide film 46a and the high dielectric constant insulating film 46b1, are formed in the region 43a for forming the high voltage system semiconductor device. As a MISFET which has such gate insulating films of a double layer structure, for example, there is a transistor described in Jpn. Pat. Appln. KOKAI Publication No. 2002-164439.
As described above, conventionally, the double layer structure constituted of the silicon oxide film and the high dielectric insulating film has been formed as the gate insulating film of the semiconductor device of the high voltage system.
Thus, because of the formation of the laminated structure of the insulating films having different dielectric constants as the gate insulating film, there have been problems of reductions in performance and reliability of the semiconductor apparatus etc., such as deterioration of mobility caused by level formation on a material interface of the double layers which constitute the gate insulating film, or fluctuation in a threshold value caused by traps formed on the material interface of the double layers by the level formation.